Today Clicker question

ISP209s10 Lecture 13 -1-

Today

• HW and Extra Credit due Thursday 8am

• Exam #2 is nominally on 3/18. I don’t want

to rush it though. I will let you know before

spring break if I need to push it back.

• Electromagnetism

• Waves


Clicker question

What will happen if

you cut this magnet

In half at the midpoint?

A) You will get two smaller magnets, one with a North pole only

and the other with a South pole only

B) You will get two smaller magnets, each one having both a North

and a South pole


Clicker question

What will happen if

you cut this magnet

In half at the midpoint?

A) You will get two smaller magnets, one with a North pole only

and the other with a South pole only

B) You will get two smaller magnets, each one having both a North

and a South pole

No isolated north or south pole (“monopole”) has ever been

seen in nature or manufactured in the laboratoryISP209s10 Lecture 13 -4-

Clicker question

True or False. You can make your own magnet from

Scratch with some wire and a battery to make electrical

Current flow thru the wire. A)True B)False


Clicker question

. A)True B)False

Moving charge creates a magnetic

field

At the microscopic

Level, “permanent”

Magnets (e.g.,


• The earth’s gravitational field surrounds the earth; it

would be there even if the moon wasn’t.

• The existence of a field in a region of space means that

if an appropriate object is in this region, it will

experience a force.

• An electric field will exert a force on a charge

• a magnetic field will exert a force on a magnet or a

moving charge.

Force Fields: A disturbance of space


Electric Field example

• If we move a test charge, q, in the vicinity of anothercharge we can make a map of the force (magnitude &direction)

• Define: Electric field E = F/q• Electric field is a vector. Its units are N/C or V/m

(volts/meter). It points in the direction of the force.

• Once we know the electric field we can calculate theforce everywhere in space: F=qE

F = electric field times charge of the object in the field

Which is underlying reality, the force or the field?ISP209s10 Lecture 13 -8-

Samples

• Electric field lines point away from positive

charge and toward negative charge.

• Charge generates an electric field. F = qE


Maxwell’s Equations Unify Electricity/Magnetism

Weird.The equations predict the existence ofsomething (a wave) that travels with speed c, thespeed of light!

Charge makes an electric field.

Moving charge makes a magnetic

field.

Changing magnetic field makes an

electric field (“Faraday’s Law”)

Magnets always have a north and a

south pole0

1

14

4

=!"

#

#$=%"

#

#+=%"

=!"

B

t

B

cE

t

E

cJ

cB

E

r

rr

rrr

r

&

&'


Faraday’s law is behind

the whole industry of

electric power

generation.

If a power source can be

made to turn a shaft in a

magnetic field,

electricity will be

produced.

Electricity generation


The Strength of the Electric Field

• Electric potential – SI unit is the Volt (V)

• Electric field is rate of change of potential

• The minus sign means that electric fields

point from + to – charge.

x

VE

!

!"=


A visual picture – Electric Potential

-

+

+q

Steep slope means a larger force.

Positive charge makes a hill

Negative charge makes a

valley.


Example: Lightning

Potential difference of 100 MV is developed between cloud and

ground. In the bolt about 5 C of charge are transferred (on average).

1 electron = 1.602E-19 C ISP209s10 Lecture 13 -14-

Sample Problem

What is the magnitude

of the electric field at:

• 0.5 m?

• 1.5 m?

• 3.0 m?

The field is 0 V/m at 0.5 m and 3.0 m since the slope is zero.

( )( ) m

V

mm

VVE 100

12

0100

x

V 1.5m)at ( =

!

!=

"

"=


Electric fields and potential

• In equilibrium the electric field in a metalconductor is zero.

• This means that inside a metal the electricpotential is flat, like the flat top of a table.

• Sitting inside a metal cage is like sitting on top ofa large, flat table. As long as you are in the center,there is no danger of falling off.

• This is why being in a car during a thunder stormis relatively safe.


Electric Field Example

Electric Field

E = -!V/!x =-(50V-90V)/1m= 40 V/m

+ means to the right in this case

Q = 0.5 µC = 0.5x10-6 C What is the magnitude of the electric

force on Q?

F = qE

F = 0.5E-6C x 40 N/C = 20E-6 N

Not asked for, but the direction is +,

to the right.


Flow of Charge - Current

• Current is the rate of flow of charge. SI

units is Ampere = 1 Coulomb/second

• Batteries are like pumps that lift charge to a

higher potential. The charge flows down the

hill to the other side of the battery.

A battery

is like a

pump.

Moving Charge

does work on the

way downV


4 Types of electric materials in Nature

• Conductor – electrons in the conduction band;electrons relatively free to flow (copper, aluminum,gold, silver)

• Insulator – no electrons is the conduction band;electrons can not flow (wood, most rubber, mostglass, most plastic)

• Semiconductor – at finite temperature, someelectrons are in the conduction band (used in mostelectronics; silicon, germanium)

• Superconductor – at very low temperature electronspair and can move freely without resistance(Niobium, Titanium, Lead)


Conductor

electrons

energy

Conduction

band

V

Electrons hit bumps, but are free to roll.

e.g., copper, iron, aluminum, silver, gold, …


Insulator

electrons

energy

Conduction

band

V

Electrons are not free to roll.

e.g., rubber, plastic, glass, wood,…


Semiconductors

V

electrons

energy

Conduction

band

light LED – light emitting diode

e.g., Silicon in computer chips


Superconductor

electrons

energy

Conduction

band

V

No resistance to flow (also no use of energy)

The magic behind magnetically levitating trains


Have a friend hold 1 end of a spring while

you “snap” the other end up and down.

A wave travels along a Slinky. But what is

traveling?

It’s the shape that travels, and it carries

energy with it.

The shape – or disturbance – is the wave, and

the Slinky is the medium through which the

wave travels.

Waves: Something else that travels


Wavelength " is the distance from any point on

the wave to the next similar point.

Frequency # (1 Hertz = 1/s) is the number of

vibrations made by any point per second.



Amplitude is the maximum height or depth of

the wave (measured from the midpoint).



Finally, the wavespeed is the speed at which the

disturbance travels through the medium.



So, what enters your eyes when you see light? In

particular, is it a particle or a wave?

Light: Particle or Wave?

Answer: It depends. More later on quantum mechanics and the

Wave-particle duality of Nature.

For now, we take the pre-quantum view that light is a wave.

We’ll see why light has wave-like features next class…

What is “waving”?

More on this later…


Wavelength and Frequency

" = 1.0 m

Wavelength Frequency = 1/period

Distance over which the wave

repeats

Number of cycles (repeats) per

second.

period = 2.0 s


The Electromagnetic Spectrum

Prentice-Hall 2005

Speed = " f

" – wavelength

f – Frequency, Hz

(1/period)(1/s)

For light

Speed c = 3.0E+8m/s

Next time we’ll see

How light can be

Understood as a

Particle (“photon”)

Documents

Today Clicker question